[PATCH macros v2] Fix XORG_WITH_XMLTO to work with xmlto >= 0.0.27

[Andreas Boll]

Starting with xmlto version 0.0.27 the return code of
  xmlto --skip-validation txt conftest.xml
is non-zero if conftest.xml is an empty file.

As a consequence the macro XORG_WITH_XMLTO returns
  "xmlto cannot generate text format, this format skipped"
and therefore libraries like libxi, libxdmcp and others won't convert
docbook XML to text format.

This changed behavior was introduced with the following change in xmlto:
  xmlto.in: use correctly exit code from xsltproc
See also: https://fedorahosted.org/xmlto/changeset/77

This patch fixes this by additionally testing xmlto with a non-empty XML
file.

More details can be found at:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=613674

Thanks to Peter Pearse, Helmut Grohne and Gaetan Nadon.

v2: To maintain compatibility with older xorg tarballs don't replace
    the original test with the empty XML file but instead add a fallback
    to additionally test with a non-empty XML file if the original test fails.
    Use the alternate solution with <x></x> to skip compatibility issues
    with different docbook versions.

Cc: Gaetan Nadon <memsize at videotron.ca>
Signed-off-by: Andreas Boll <andreas.boll.dev at gmail.com>
---
I've successfully tested this patch to build libxi with xmlto 0.0.26, 0.0.27
and 0.0.28.

To test this patch on a platform with only docbook version 5 installed
I've replaced docbook-xml with docbook5-xml. On such a setup the macro
XORG_WITH_XMLTO detects xmlto correctly though it fails later in the build
with:

xmlto: /«PKGBUILDDIR»/build/man/XAllowDeviceEvents.xml does not validate
(status 3)
xmlto: Fix document syntax or use --skip-validation option
I/O error : Attempt to load network entity
http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
/«PKGBUILDDIR»/build/man/XAllowDeviceEvents.xml:2: warning: failed to
load external entity
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
D DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
                                                       ^
I/O error : Attempt to load network entity
http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
warning: failed to load external entity
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
validity error : Could not load the external subset
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
Document /«PKGBUILDDIR»/build/man/XAllowDeviceEvents.xml does not
validate

This build failure is expected since docbook version 5 is not backward
compatible to version 4 and XORG_WITH_XMLTO doesn't check for a specific
docbook version.

 xorg-macros.m4.in | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/xorg-macros.m4.in b/xorg-macros.m4.in
index 9b3c503..2ed7837 100644
--- a/xorg-macros.m4.in
+++ b/xorg-macros.m4.in
@@ -426,13 +426,24 @@ m4_ifval([$1],
 fi])
 
 # Test for the ability of xmlto to generate a text target
+#
+# NOTE: xmlto 0.0.27 or higher return a non-zero return code in the
+# following test for empty XML docbook files.
+# For compatibility reasons use the following empty XML docbook file and if
+# it fails try it again with a non-empty XML file.
 have_xmlto_text=no
 cat > conftest.xml << "EOF"
 EOF
 AS_IF([test "$have_xmlto" = yes],
       [AS_IF([$XMLTO --skip-validation txt conftest.xml >/dev/null 2>&1],
              [have_xmlto_text=yes],
-             [AC_MSG_WARN([xmlto cannot generate text format, this format skipped])])])
+             [# Try it again with a non-empty XML file.
+              cat > conftest.xml << "EOF"
+<x></x>
+EOF
+              AS_IF([$XMLTO --skip-validation txt conftest.xml >/dev/null 2>&1],
+                    [have_xmlto_text=yes],
+                    [AC_MSG_WARN([xmlto cannot generate text format, this format skipped])])])])
 rm -f conftest.xml
 AM_CONDITIONAL([HAVE_XMLTO_TEXT], [test $have_xmlto_text = yes])
 AM_CONDITIONAL([HAVE_XMLTO], [test "$have_xmlto" = yes])
-- 
2.1.4